<?xml version="1.0" standalone="yes"?>
<Paper uid="C92-2108">
  <Title>Preventing False Temporal Implicatures: Interactive Defaults for Text Generation*</Title>
  <Section position="6" start_page="0" end_page="0" type="concl">
    <SectionTitle>
* Causal Law
</SectionTitle>
    <Paragraph position="0"> If the clauses ,~ and fl are discourse related, anti c~ and /7 describe respectively the events el of x falling aud e2 of y pushing x, then normally e~ caused e I * Consider the ca.qc where S intends to convey the proposition that John's pushing Max caused the latter to fall. Suppose S has a KB which will allow her to generate the description in (9), among others.</Paragraph>
    <Paragraph position="1"> (9) Max fell. John pushed llim.</Paragraph>
    <Paragraph position="2"> We have argued that this text is coherent, precise and reliable for S because tile causal law (about the usual causal relation between pushings and failings) is more specific than the linguistic ride (Narration). But since H lacks the causal law, (9) will trigger a different inference pattern in H; one in which Narration wins after all. S must block this pattern by changing the utterance; she has eascntially two options. If clause order is kept fixed, then ,5' could shift, tense into the pluperfect im in (10); or else S can insert a clue word, such as because, into tile surface form, to generate (11):  (10) Max fell. John \]lad pushed him.</Paragraph>
    <Paragraph position="3"> (11) Max fell because John pushed him.</Paragraph>
    <Paragraph position="4">  The success of tile latter tactic requires ,'5' and H to nmtually know a new linguistic rule, more specific ttlan Narralion, such as the following: 4</Paragraph>
    <Paragraph position="6"> ment is (r because fl, then normally tile ewmt lie scribed in ~ caused that described in/7.</Paragraph>
    <Paragraph position="7"> On tile otimr hand, if clause order is not taken to be fixed, then 5' can simply reorder (9): (12) John pushed Max. Max fell.</Paragraph>
    <Paragraph position="8"> So, when 5&amp;quot; bclicves H lacks the relevant causal law, 5&amp;quot; can simply reorder, and let Na~'atiou do the rest. However, recalling the above discussion, in some cases a discourse structure that invokes Explanation is better than one that invokes Narration. So simply reordering events and letting the rule for Narration achieve tile correct inferences won't work successfidly in all cases. Furthermore, recaning tile iliscussion about states and causation above, it becomes appar.</Paragraph>
    <Paragraph position="9"> ent that this tactic of always letting Narration do tile work will lead to problenls with texts like (3)  and (4).</Paragraph>
    <Paragraph position="10"> (3) Max opened the dnor. The roonl was pitcll (lark. (4) Max switched off the light. The room was pitch  dark.</Paragraph>
    <Paragraph position="11"> q'lle reason is that, in the absence of the causal law which relates light switching to darkness, (4) will be analysed exactly as (3), giving the wrong result. A solution would be to replace the state expression with all event expression: (4/) Max switched off the light. The room wen~ pitch dark.</Paragraph>
    <Paragraph position="12"> 4This is a pragmatic, rather than semantic rule; it's not obvious tll~t this is tile best choice of representation. An obviolts alternative is to introduce further clue words, and appropriate linguistic rules for reasoning about them. This means exploiting linguistic knowledge to overemne tile gaps in H's world knowledge. This tlelps explain tile observation that texts which (lescribc events ill reverse to temporal order, without marking the reverse, may bc quite rare. It's easy enough 1o interpret such texts, when we have the all: l&gt;ropriate WK. lhlt if a considerate speaker or writer ha~ reason to believe that some or all of her audience lacks that WK, then she will either avoid such descriptive reversals, or mark them with thc type of clues we have discussed.</Paragraph>
    <Paragraph position="13"> p is a fact in the Krl We now turn to the case where p is a fact about tim Kn which S knows and which S knows H lacks. Suppose that p asserts a causal relation between two events (lilt does not represent an excelltion to any (lefi;p~qible causal preferences, and that S wishes to convey tile information that p. Then S can simply state p by exploiting H's available LK. Clue words may not be needed.</Paragraph>
    <Paragraph position="14"> For example, if p is ttle fact that Max stood up and ttlen John greeted him, S can tell H ttlis by uttering (8); Na*~'alion will make (8) reliable and precise for L/.</Paragraph>
    <Paragraph position="15">  (8) Max stood Ul). John greeted him.</Paragraph>
    <Paragraph position="16"> Similarly, if p is tile fact that Max opened tile door, and wtlile this was going on tile room was pitch dark, then (3) is reliable and precise for 11 : (3) Max opened tim door. The room was pitch dark.  But what if p asserts a causal relation between two events that violates a dcfe~mible causal preference that H has? Snppose p asserts that Max's fall iumlediately preceded aolln's puslling hinl. And suppose that S knows that H has tile defeasible causal law mel~tioned above, but lacks p. Then neitller (9) nor (12) are reliable for //, indicating that S cannot  generate all atomic text, to assert p.</Paragraph>
    <Paragraph position="17"> (9) Max fell. John pushed him.</Paragraph>
    <Paragraph position="18"> (12) Jobn pushed Max. lie fell.</Paragraph>
    <Paragraph position="19">  tf wouhl interpret (9) a~ an explanation; and (12) as a narrative, for nothing will eontliet with Narra(inn in that case: tile causal preference for pushings causing failings would simply reinforce the temporal structure imposed by Narration. The obvious option is to nlove from (9) to 113); anotber option is to recruit tile pluperfi:et, ms ill 114); note that 115) is not  a sohttion, since so can be read evidentially.</Paragraph>
    <Paragraph position="20"> (13) Max fi~ll. And then John pushed Ilint.</Paragraph>
    <Paragraph position="21"> (14) J(?a~ pushed Max. lie had fallen.</Paragraph>
    <Paragraph position="22"> (15) Max fell. So John pushed him.</Paragraph>
    <Paragraph position="23">  The seed it) utter (13) rather tllan (9) explains why it c~Ltl be necessary to use and then, even tllough th,'~ thll-stop is always available and, by Narration, has the default effect of temporal progression. So, ill general, one might wish to paraphrase Joshi et all if a relation CtUl be defeasibly referred to hohl between two eventualities, and S' wants solnething dilferent, it is essential to mark the desired relation with sometlling strunger.</Paragraph>
    <Paragraph position="24"> ACRES DF. COL1NG-92, NANTES, 23-28 AOt'rl 1992 7 2 5 PROC. OF COLING 92, NANq ES. AUG. 23-28, 1992 Case 2: 6: knows p but isn't sure if H does In general, S will have only partial knowledge about H's beliefs. This has its drawbacks.</Paragraph>
    <Paragraph position="25"> p is a defeasible causal preference Suppose that S isn't sure whether or not H believes the defea-sible causal law relating falling and pushing. Then there are at least two ways in whicb S's model of H's knowledge can be expanded to a complete statement of H's knowledge. The first, B1, contains the causal law. The second, B2, does not. Now uppose that S wishes to convey the proposition that John's pushing Max caused Max to fall. Then, if S assumes H's knowledge corresponds to Bl, then H will find a reliable interpretation for (9).</Paragraph>
    <Paragraph position="26"> (9) Max fell. John pusbed him.</Paragraph>
    <Paragraph position="27"> On the other hand, if S assumes that H's knowledge corresponds to BT, then H will interpret (9) in an undesirable way, witt, the falling preceding the pushing; as we said before, Narration would win.</Paragraph>
    <Paragraph position="28"> Under this model, S isn't sure how H will interpret (9), because S doesn't know if H's knowledge correo sponds to B! or B2. Hence the ambiguity of(9) manifests itself to the generator S, if not to the hearer H, because S doesn't haw. ~ sufficient information about H to predict wbich of the two alternative temporal structures H will infer for (9). This is slightly different to the previous case where S actually knows H lacks the causal law, making (9) unreliable.</Paragraph>
    <Paragraph position="29"> &amp;quot;lb avoid uttering unreliable text, S will have to utter something other thmJ (9). Indeed, it may be  possible for S not to worry about tim ambiguity of (9) at all, if some 'safe' strategy can bc tbund that  would guide S's expansion of H's knowledge in a way that wmdd ensure the generation of reliable text for H. A plausible strategy for S's reasoning about H would he the following: if S isn't sure whether or not H knows p, then assume H doesn't know p. On the face of it this seen~s plausible. But just how safe is it? We state it in terms of B+(H) and B-(H): * lfp q B+(H) and p q B-(H), assume p E B-(H) and generate-and-test under this assumption.</Paragraph>
    <Paragraph position="30"> But this won't work in general. If S wants to convey a violation of the causal law p, but H actually believes p, tben the strategy will suggest the use of (9), which will actually be unreliable for H.</Paragraph>
    <Paragraph position="31"> In fact, there is no safe strategy, save tim ouc where S considers several alternative expansions of H's knowledge. As a result, ambiguity of text will manifest itself to S in certain cases, because of her partial knowledge of H. This is perhaps somewhat surprising. Nonmonotonic reasoning is designed as a medium for reasoning witb partial kuowledge. And yet here we have shown S cannot maintain textual reliability on the basis of a partial statement of H's KB, even if nonmonotonic inference is exploited.</Paragraph>
    <Paragraph position="32"> p is a fact about the KB: Ambiguity Suppose that 5' wants to convey the information that Max's fall immediately preceded John pushing \]tim, and suppose S knows that H knows the causal law, but S doesn't know for sure if H knows already that Max fell before John pushed him. Then, for similar reasons as those mentioned earlier, S isn't sure if (9) is reliable or not.</Paragraph>
    <Paragraph position="33"> (9) Max fell. John pushed him.</Paragraph>
    <Paragraph position="34"> '17o be sure that text is reliable in this case, .q will again have to exploit linguistic knowledge; for exampie, by uttering (13) instead of (9).</Paragraph>
    <Paragraph position="35"> (13) Max fell and then John trashed him.</Paragraph>
    <Paragraph position="36"> Case 3: H as advisor, S as pupil Suppose that for a certain proposition p, p C/ B(S), p q B4&amp;quot;(H) and p f/ B-(H). This corresponds to H potentially knowing more about p than S, but S not knowing what more. That's pretty much the position of the tutee in a tutorial dialogue, and the advice-taker in an advisory dialogue.</Paragraph>
    <Paragraph position="37"> Case 4: S thinks that H is mistaken Suppose that p f\[ B(S) and p E B+(H). Then S doesn't believe p even though site's aware H does. This implies that 5' thinks H is mistaken in believing p.</Paragraph>
    <Paragraph position="38"> The fact that p q B(S) and p E B+(tt) could entail that a text that's reliable for S isn't for H. For example, suppose that H believes, by some weird perception of social convention, that there is a defeat sible cansal preference that greetings cause standing ups. Suppose tbat S wants to describe the situation where Max stood up and then John greeted him (i.e. an exception to H's causal preference). Then this is like the exception case above concerning falling and pushing: (16) is reliable for S but not for H.</Paragraph>
    <Paragraph position="39"> (16) Max stood up. John greeted him.</Paragraph>
    <Paragraph position="40"> Again, S could compensate for this by explicitly marking the temporal relation. Alternatively, the fact that p C/ B(S) and p E B+(H) could entail that a text that's unreliable for S is reliable for H. Again, let p be the causal law that says that greetings cause standing ups. But this time suppose that S wants to describe the situation where John's greeting Max caused him to stand up. So this time, S wants to describe an instance of the causal law. Then both (16) and (17) are reliable for H, but only the latter is reliable for S.</Paragraph>
    <Paragraph position="41"> (17) John greeted Max. tie stood up.</Paragraph>
    <Paragraph position="42"> (16) is unreliable for S. Arguably, it wouldn't be in the set of possible linguistic realisatious, but only if this set is assumed to be characterised by what S finds reliable. But we bare no argument for this assumption, and so we don't make it.</Paragraph>
    <Paragraph position="43"> Conclusions Ilere, we summarise the current state of the model, and briefly discuss two of its limitations.</Paragraph>
    <Paragraph position="44"> We admitted that that job of defeasible reasoning in generation could be very general; but ttlat we were going to look at it in the context of the Interactive Defaults strategy. ID applies to the candidate utterances (or tile space of utterances), and criticises the utterances (or the space), producing better utterances, or a smaller space. The notion of logical Ac-rEs DE COLING-92, NArm.;s, 23-28 Ao~r 1992 7 2 6 PROC. OF COLING-92, NANTES, AUG. 23-28. 1992 consequence supported by CE was used to make precise how utterances are constrainted by m. Crucially, we used Defensible Modus Ponens and the Penguin Principle. The grounds for criticism were the tempo. ral ramifications of the utterance; if it was incoherent for //, unreliable for H or dangerously ambiguous (for 5'), it was bad.</Paragraph>
    <Paragraph position="45"> One limitation of the model is that, although it permits reasoning about the knowledge or beliefs of interlocutors, it neglects their goals and intentions to do actions. ID does not deal with the phenomena which motivate the work following Cohen and Perrault \[1979\] and Allen and Perrault \[1980\], (cf. Cohen, Morgan and Pollack \[1990\]). In particular, ID does not let S take into account those inferences H will make in attempting to ascribe a plan to S.</Paragraph>
    <Paragraph position="46"> Hobbs et al \[1990:44-45\] argue that inferences leading to plan recognition are less significant in interpreting long written texts or monologues, llence, it might be argued that the generation of such discourses need not give H's plan recognition particular weight. Nonetheless, ID is incomplete, to the extent that such inferences inflncncc discourse generation. Secondly, discourse structure and temporal structure have become somewhat detached. Sometimes, it's only the causal-temporal structure derivable front the candidate that is being criticized. It may there fore be thought that the discourse structure is aa idle wheel as things stand, and should be either eliminated (el. Sibun \[1992\]), or bc trusted with a greater share of the work, enriching the discourse with useful clue words (cf. Scott and Souza \[1990\]). Our tentative view is timt tire latter view is plausible, and anyway is closer to the idea of generation by defensible reasoning, canvassed early on.</Paragraph>
    <Paragraph position="47"> The |D strategy examined here seems to involve a lot of hard work generating simple eamlidates which almost always require debugging. It would be preferable if we could do this work in advance, by defanlt. The alternative is explored in Lascaridcs and Oberlander \[1992b\], in which we abduce discourse structures from event structures, mid then interleave deduction and abduction to derive linguistic realisations. But in turning to the more global approach, we should not lose sight of the fact that simple texts are sometimes best. (2) illustrates this point: the rhetorical relations inferred aren't syntactically marked, arid yet the text is more natural than (1), where the relations are marked. As might be expected, there seems to be a trade-off between the naturalness of the output and its computational cost.</Paragraph>
  </Section>
class="xml-element"></Paper>